The present invention relates to a secondary battery using a non-aqueous electrolyte.
Recently, AV devices and electronic devices such as personal computers of portable or cordless type have been rapidly developed, and demand for secondary batteries which are small in size, light in weight and high in energy density is great as electric sources for driving these devices. Among them, non-aqueous electrolyte batteries using lithium as negative electrode active materials are expected much as batteries having high voltage and high energy density.
In the above batteries, lithium-containing metal oxides which show a voltage on the order of 4 V for lithium, such as LiCoO2, LiNiO2 and LiMn2O4, are used as positive electrode active materials, and carbon materials and the like which can intercalate or deintercalate lithium are used for negative electrodes.
As electrolytes of these non-aqueous electrolyte batteries, there are used non-aqueous solvents, for example, cyclic esters such as ethylene carbonate and propylene carbonate, cyclic ethers such as tetrahydrofuran, non-cyclic ethers such as dimethoxyethane, and non-aqueous esters such as dimethyl carbonate and methyl propionate, and mixed solvents thereof. Among them, non-cyclic esters are often used because they can provide electrolytes of low viscosity and high conductivity.
At present, the above batteries have a wide variety of uses for not only portable type information terminals (such as portable telephones and notebook type personal computers), but also electric cars and power storage, and use environments therefor become increasingly severer. Especially, considering the use as electric sources for electric cars, batteries are exposed to high-temperature environment of higher than 80xc2x0 C. in summer, and batteries having high reliability even at such severe environmental temperatures are desired.
In these secondary batteries, non-aqueous organic solvents which do not undergo electrolysis even at high voltages are used in an attempt to provide batteries having high reliability. However, deterioration of discharge characteristics is caused in the environment of high temperatures, and, in the worst case, leakage of electrolyte may occur.
As a result of investigation conducted by the inventors for the purpose of providing excellent batteries having high reliability even when they are stored at high temperatures, they have found an interesting phenomenon on behavior of oxidative decomposition of electrolyte. That is, an oxidative decomposition of electrolyte takes place on the surface of lithium-containing metal oxide which is a positive electrode active material in the environment of high temperatures, but the decomposition product is usually a low molecular compound and is released out of the electrolyte system as a gas or dissolves in the electrolyte. Therefore, it has been found that even if the oxidative decomposition of electrolyte occurs, the active points of the positive electrode active material are not poisoned and keep high activity, and, hence, the oxidative decomposition reaction of the electrolyte continuously proceeds to result in exhaustion of the electrolyte, increase of internal pressure due to evolution of a decomposition gas in a large amount, and leakage of the electrolyte.
It has been found that especially when non-cyclic esters such as dimethyl carbonate, diethyl carbonate and ethylmethyl carbonate are used as non-aqueous solvents of electrolyte, an ester interchange reaction proceeds in an environment of high temperatures to produce methoxy group (CH3Oxe2x80x94) or ethoxy group (CH3CH2Oxe2x80x94) in the course of the reaction, which attacks active points of the positive electrode as a nucleophilic reagent to cause further progress of the decomposition reaction of the electrolyte.
The present invention which is based on the new knowledge on the behavior of oxidative decomposition of electrolyte relates to a non-aqueous electrolyte secondary battery using a lithium-containing oxide as a positive electrode active material, a material capable of absorbing and releasing lithium as a material of negative electrode, and a non-aqueous solvent containing at least one non-cyclic ester as a non-aqueous electrolyte, characterized in that an oxidation resistance improving agent is added to the non-aqueous electrolyte, and, thus, increase of internal pressure or leakage of electrolyte can be effectively inhibited by improving the oxidation resistance of the electrolyte.